Ultrasonic tissue characterization of myocardium is a promising application of ultrasound to the diagnosis of cardiac disease. Several problems need to be resolved before tissue characterization can be used clinically. The long-term goal of this research is to develop insight into the interaction of ultrasound with normal and ischemic myocardium so that it can be applied clinically to detect and quantitate myocardial infarction, as well as follow structural and functional recovery of myocardium during interventions to reduce infarct size. Studies will be performed in anesthetized open-chest, conscious dogs, and in patients with myocardial infarction. Three specific problems will be addressed: 1) delineation of mechanism(s) of ultrasonic backscatter directly by examining myocardial tissue with Scanning Laser Acoustic Microscope, and indirectly by testing the hypothesis that since cardiac cycle variability is dependent on myocardial contraction, it will also vary with changes in heart rate, preload, afterload, and inotropic state; 2) role of ultrasonic backscatter to assess the viability of ischemic myocardium and quantify infarct size in open-chest dogs; and 3) to assess the accuracy of tissue characterization in conscious dogs and patient with myocardial infarction. Ultrasonic backscatter will be measured under conditions of variable heart rate, preload, afterload, and inotropic state in anesthetized open-chest dogs. Viability of ischemic myocardium will be assessed by acute occlusion of the left anterior descending coronary artery for variable periods followed by reperfusion. Histochemical analysis of infarcted tissue will be compared to ultrasonic estimates of infarction. Tissue biopsies of normal, ischemic, and infarcted myocardium will be examined with an acoustic microscope. Ultrasonic backscatter will also be measured through the chest wall in conscious dogs with chronic myocardial infarction. Clinical studies will be performed, initially, intraoperatively in patients undergoing elective open heart surgery. Subsequently ultrasonic studies will be performed in patients with acute and chronic myocardial infarction. These studies will provide insight in to the mechanism(s) of ultrasonic backscatter and its variability with cardiac cycle, as well as permit clinical application of a non-invasive method to detect and estimate infarct size.